Emergence of two distinct regimes in phonon-induced non-equilibrium magnetization dynamics

Driving infrared (IR)-active phonons to large amplitudes to enable non-equilibrium crystal lattice distortions, known as non-linear phononics, can initiate phase transitions along non-thermal pathways, providing transient control of various material properties beyond the equilibrium limits. Yet, how these non-thermal lattice-driven states evolve and thermalize remains unresolved. Here, we explore the crossover from non-thermal to thermal magnetization dynamics in dysprosium orthoferrite (DyFeO₃), driven by the resonant excitation of IR-active phonons. Using mid-infrared light pulses, we induce a transition from the collinear antiferromagnetic to the weakly ferromagnetic (WFM) phase, resulting in the emergence of net magnetization. Time-resolved single-shot magneto-optical imaging across multiple timescales reveals two distinct regimes. First, magnetization emerges as a spatially uniform state whose direction is controlled by the pump polarization, indicative of a non-thermal mechanism driven by non-linear phononics. On longer timescales, this state relaxes into a multidomain pattern that is insensitive to the pump polarization, consistent with thermal equilibration. The crossover occurs on a timescale of about 200 ps, far exceeding the IR phonon coherence time and consistent with the spin-lattice relaxation time in the WFM phase. These findings provide direct temporal and spatial fingerprints of non-linear-phononics-driven magnetic phase control, defining intrinsic limits for reversible ultrafast manipulation of magnetic order.